allow:
1\to let someone do or have something, or let something happen
allow somebody/something to do something
----don't allow your problems to dominate your life
这个是longman中allow的第一个解释,我觉得allow his grades to drop,更倾向于是let something happen的意思。
楼主如果还有什么疑惑,可以自己去查longman或是其他英英字典,可能会有比较好的帮助
^-^